Jason S. Link

Ecological Considerations in Fisheries Management: When Does it Matter?

Abstract Ecological processes are often not explicitly included in fisheries models and management, but have the potential to be a significant influence on fish stocks. I examine cases from ecosystems around the world where predation, competition, environmental regime shifts, and habitat alteration have altered population dynamics, stock abundance, and community composition. These cases demonstrate the importance of ecological processes in the regulation of fish populations. Fisheries managers and scientists have long appreciated the importance of quantifying ecological processes relative to fishing, but recognizing the consequences of ignoring these considerations and developing a feasible set of widely used tools to implement these considerations has been lagging. Very simply, given what we know about the importance of these processes, particularly at low stock abundances, we would be wise to address them. I conclude with a proposed set of questions that need to be addressed in order to more widely and ...

Pathways between Primary Production and Fisheries Yields of Large Marine Ecosystems

The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems.

Changes in piscivory associated with fishing induced changes to the finfish community on Georges Bank

There are many ecologically and commercially important piscivores in the Georges Bank fish community. There has been a noticeable shift in the abundance of these predators during the last four decades which is generally attributed to fishing pressure. Although many fish persist as piscivores in this system, not only has their relative abundance changed but their size composition has also changed. Both factors influence the total magnitude of piscivory by these fish. To ascertain the cascading effects of fishing on fish predation, we examined relative abundance, total stomach contents, diet composition, consumption rates, and total food consumption of fish for several species across the time period. The proportion of the diet comprised by fish for piscivores on Georges Bank has remained remarkably consistent. However, the composition of specific fish prey has changed across the time series. Total fish consumption generally tracked predator abundance, but two species demonstrated compensating consumption rates as larger size classes declined. Contrary to our initial hypothesis, total fish consumption by six major predators has remained generally constant despite changes in predator size, structure and abundance. One reason for constant systemic piscivory is the dominant piscivore in this ecosystem has shifted from cod to spiny dogfish. We assert that a major effect of intense fishing pressure is a shift in energy flow for marine ecosystems.

Myths that Continue to Impede Progress in Ecosystem-Based Fisheries Management

Ecosystem-based fisheries management has been perceived as something desirable but pragmatically unachievable due to several impediments identified earlier during its implementation phase. Over the years, many of these impediments have been resolved but not well communicated to stakeholders, managers, scientists, and policymakers. As a result, several past impediments to implementing ecosystem-based fisheries management have taken on a mythical status. Here we identify six common myths, address why they in fact no longer impede ecosystem-based fisheries management, and propose solutions for moving forward. We assert that these myths need not continue to exist and that improved approaches for fisheries are indeed feasible.

Ecological Interactions between Elasmobranchs and Groundfish Species on the Northeastern U.S. Continental Shelf. I. Evaluating Predation

Abstract Elasmobranchs are an important component of the northeastern U.S. continental shelf ecosystem in terms of both biomass and abundance. It has been hypothesized that because of their high biomass these species have a significant negative impact on commercially valuable groundfish via ecological interactions. One of these hypotheses posits that elasmobranchs directly remove such groundfish by predation. We analyzed data from the stomachs of 40,756 spiny dogfish Squalus acanthias, 3,806 smooth dogfish Mustelus canis, 17,618 little skate Raja erinacea, 11,593 winter skate R. ocellata, and 2,571 thorny skate R. radiata collected in bottom trawl surveys conducted principally during spring and autumn 1973–1998 in waters from Cape Hatteras, North Carolina, to Nova Scotia. Except for silver hake Merluccius bilinearis, most groundfish are not common in the diets of the elasmobranchs. Estimates of consumption indicate that both the number and the total biomass of the groundfish that are removed are generally...

An Ecosystem Approach for Assessment Advice and Biological Reference Points for the Gulf of Maine–Georges Bank Atlantic Herring Complex

Abstract The biomass of Atlantic herring Clupea harengus in the Gulf of Maine–Georges Bank (GOM–GB) region declined sharply in the mid- to late 1970s, when fishery harvests ranged from 300 to 400 kilo–metric tons (kt) per year. During this same period, the consumption of Atlantic herring by fish and marine mammal predators was very large. Consumption has greatly exceeded landings from 1985 to the present. Previous research showed that four groups of predators (demersal fishes, marine mammals, large pelagic fishes, and seabirds) together annually consumed over 200 kt of Atlantic herring during the 1970s and over 300 kt during the late 1990s and early 2000s. Our analyses indicate that Atlantic herring predation mortality rates are related to the abundance of both Atlantic herring and predators. Predation mortality rates were relatively low during the 1960s, when Atlantic herring were abundant, but increased in the late 1970s and early 1980s, when Atlantic herring declined. Predation mortality rates declined...

Silver hake tracks changes in Northwest Atlantic circulation

Recent studies documenting shifts in spatial distribution of many organisms in response to a warming climate highlight the need to understand the mechanisms underlying species distribution at large spatial scales. Here we present one noteworthy example of remote oceanographic processes governing the spatial distribution of adult silver hake, Merluccius bilinearis, a commercially important fish in the Northeast US shelf region. Changes in spatial distribution of silver hake over the last 40 years are highly correlated with the position of the Gulf Stream. These changes in distribution are in direct response to local changes in bottom temperature on the continental shelf that are responding to the same large scale circulation change affecting the Gulf Stream path, namely changes in the Atlantic meridional overturning circulation (AMOC). If the AMOC weakens, as is suggested by global climate models, silver hake distribution will remain in a poleward position, the extent to which could be forecast at both decadal and multidecadal scales.

UNDERAPPRECIATED SPECIES IN ECOLOGY: “UGLY FISH” IN THE NORTHWEST ATLANTIC OCEAN

Species shifts and replacements are common in ecological studies. Observations thereof serve as the impetus for many ecological endeavors. Many of the species now known to dominate ecosystem functioning were largely ignored until studies of those underappreciated species elucidated their critical roles. Recognizing the potential importance of underappreciated species has implications for functional redundancies in ecosystems and should alter our approach to long-term monitoring. One example of an applied ecological system containing species shifts, underappreciated species, and potential changes in functional redundancies is the topic of fisheries. The demersal component of many fish communities usually consists of high-profile and commercially valuable species that are targets of fisheries, plus a diverse group of lesser known species that have minimal commercial value and focus. Yet ecologically these traditionally nontargeted species are often a major biomass sink in marine ecosystems and can also be critical in the functioning of bentho-demersal food webs. I examined the biomass trajectories of several species of skates, cottids, lophiids, anarhichadids, zooarcids, and similar species in the northeast U.S. Atlantic ecosystem to determine whether their relative abundance has changed across the past four decades. Distribution and stomach contents of these species were also evaluated over time to further elucidate the relative importance of these species. Landings of these underappreciated bentho-demersal fish were also examined in comparison to those species that historically have been commercially targeted. Of particular emphasis was the evaluation of evidence for sequential stock depletion and the ramifications for functional redundancy for this ecosystem. Results indicate that some of these fish species are now the dominant piscivores, benthivores, and scavengers in this ecosystem. These formerly under-studied species generally have either maintained a consistent population size or have increased in abundance (and expanded in distribution) over the past several decades. Nontraditionally targeted fish species are an often overlooked but important component of bentho-demersal fish communities. Implications for the energy flow and resilience specifically for future fisheries and generally for harvesting biological resources are significant, remaining critical issues for the worlds ecosystems.

Role of egg predation by haddock in the decline of an Atlantic herring population

Theoretical studies suggest that the abrupt and substantial changes in the productivity of some fisheries species may be explained by predation-driven alternate stable states in their population levels. With this hypothesis, an increase in fishing or a natural perturbation can drive a population from an upper to a lower stable-equilibrium population level. After fishing is reduced or the perturbation ended, this low population level can persist due to the regulatory effect of the predator. Although established in theoretical studies, there is limited empirical support for predation-driven alternate stable states in exploited marine fish populations. We present evidence that egg predation by haddock (Melanogrammus aeglefinus) can cause alternate stable population levels in Georges Bank Atlantic herring (Clupea harengus). Egg predation by haddock explains a substantial decoupling of herring spawning stock biomass (an index of egg production) from observed larval herring abundance (an index of egg hatching). Estimated egg survival rates ranged from <2–70% from 1971 to 2005. A population model incorporating egg predation and herring fishing explains the major population trends of Georges Bank herring over four decades and predicts that, when the haddock population is high, seemingly conservative levels of fishing can still precipitate a severe decline in the herring population. These findings illustrate how efforts to rebuild fisheries can be undermined by not incorporating ecological interactions into fisheries models and management plans.

Accounting Explicitly for Predation Mortality in Surplus Production Models: An Application to Longfin Inshore Squid

Abstract One approach to better account for ecosystem considerations in fisheries science is to incorporate ecological interactions into conventional stock assessment models. The longfin inshore squid Loligo pealeii is one of two squid species of ecological and commercial significance in the northwest Atlantic Ocean. A surplus production model with quarterly time steps was fitted to longfin inshore squid total removal (fishing and predation removal) and tuned with fishery-dependent, fishery-independent, and predation-dependent indices to examine the effect of incorporating predation into a single-species model. Total consumption of squid by all predatory fish exceeded the landings in most years of this analysis. The model output indicated that biological reference points for longfin inshore squid differ considerably when predation removals are included. It appears that by not including predation, the model underestimates stock biomass and overestimates fishery surplus production. Short-term stochastic pro...